Human monocytes have been employed in in vitro assay systems to determine the macromolecular basis for their function. Monocytes have been studied in their unactivated state, following muramyldipeptide activation, and following activation with poly IC/LC; unactivated cells neither release interferon nor fibroblast growth factor, muramyldipeptide stimulates monocytes to release enhanced amounts of fibroblast growth factor, and poly IC/LC only stimulates interferon production by human monocytes. When the messenger RNA from these three distinct activation states of human monocytes was analyzed for Alpha-interferon gene expression, unactivated monocytes were found not to synthesize interferon messenger RNA. In contrast, poly IC/LC-stimulated monocytes synthesize three molecular weight forms of the interferon message at 1.0 kb, 2.5 kb, and 7.5 kb. Muramyldipeptide-stimulated monocytes synthesize only a 2.5 kb interferon messenger RNA; the synthesis of this message appears to be associated with an intracytoplasmic form of interferon activity. This technology has potential clinical application for monitoring the gene expression events associated with BRM administration in cancer patients.